1. Field of the Invention
This invention relates to a method of manufacturing R--Fe--B type sintered permanent magnets. The methylcellulose and/or the agar and water mixture as a binder which induces a sol-gel reaction at a specified temperature with a R--Fe--B type alloy pulverized powder is injection molded in a magnetic field; and after the obtained molded body is dehydrated and debinded, the molded body is sintered. Thus, this invention provides a method of manufacturing a R--Fe--B type sintered magnet which controls the amount of residual carbon and oxygen in the sintered body, improving the moldability of injection molding while preventing the degradation of magnetic characteristic, and which can provide a three-dimentionally complex shaped sintered magnet.
2. Description of the Prior Art
Today, it is required to have smaller and lighter as well as high performance small motors and actuators for household appliances, computer peripherals, and automobiles, etc. Also, it is not only required to have smaller, lighter, and thinner magnet material, but it is also required to have magnet material with a three dimensionally complex shaped product with installation of a concave-convex magnet surface at a specified place and with a through hole, etc.
As high performance permanent magnets, R--Fe--B type sintered permanent magnets (U.S. Pat. No. 4,770, 223, JP-A-59-46008, JP-B- 61-34242) and a R--Fe--B type bond magnet (U.S. Pat. No. 4,902,361) were proposed.
Since the above R--Fe--B type permanent magnet as well as R--Fe--B type bond magnet usually require compression molding in the magnetic field during a manufacturing process, only a simple shaped molded body is obtained. However, in order to respond to today's requirements to have various shapes, it is proposed to study an injection molding method, which has been widely used in many engineering fields, as a method to manufacture the above R--Fe--B type sintered magnet. For example, a manufacturing method of a R--Fe--B type sintered permanent magnet (J P-A-61-220315, JP-A-62-252919, JP-A-64-28303) is proposed. An alloy powder which is obtained by pulverizing a R--Fe--B type alloy ingot and a binder which contains thermoplastic resin such as polyethylene and polystyrene, etc. as kneaded and injection molded; after the debinder treatment, the molded body is sintered to obtain the magnet. Also, a manufacturing method of a R--Fe--B type sintered permanent magnet which employs an injection molding method (JP-A-64-28302) utilizing paraffin type wax as a binder is proposed.
However, generally, intermetallic compounds containing a rare earth element (R) are likely to react with elements such as O, H, C, etc., and when binders such as thermoplastic resin and paraffin wax, etc. that are used in the above injection molding method are added to a R--Fe--B type alloy powder ad kneaded, the carbon and oxygen content usually increases due to the reaction with R. Thus, even after injection molding, the debinder treatment, and sintering, the considerable amount of carbon and oxygen remain in a sintered magnet. This results especially in degradation of magnetic characteristics, and remains an obstacle to application of a complex shaped product by injection molding to magnet parts.
Also, the above mentioned binder which is utilized in the usual the injection molding method is mixed with an alloy powder and heated to the melting point which is around 100.degree. C..about.200.degree. C. to melt the binder in the injection molding machine. Since the curie temperature (Tc) of R--Fe--B type permanent magnets is about 300.degree. C..about.350 C., it is difficult to orientate an alloy powder to the magnetizing direction when it is heated close to the curie temperature. Also, there was a problem of requiring a large magnetizing current in orientation.
Therefore, having studied binders with low melting points; hitherto, as a binder in the compression molding for Co type super alloy powder for injection molding, a composition which comprises 1.5.about.3.5 wt % methylcellulose in the said alloy powder and a specified amount of additives, glycerin and boric acid, is proposed (U.S. Pat. No. 4,113,480). Also, as binder for the injection molding for Y.sub.2 O.sub.3 --ZrO.sub.2 and alumina powder, a mixture of 10.about.50 wt % agarose, agar in the said alloy powder, and to which deionized water and glycol are added is proposed (U.S. Pat. No. 4,734,237). Furthermore, as a binder for injection molding of alloy powder for tools, a special composition wherein water, plasticizers such as glycerine, etc., lubricants and mold releasing agents such as wax emulsion, etc. are added to 0.5-2.5 wt % methylcellulose was proposed (JP-A-62-37302).
However, in the above mentioned binder of which the main ingredients are methylcellulose and agar, in order to maintain the required fluidity and molding body strength, a relatively large amount as described above is used. Also, since it is necessary to add the equal amount of binder additives, for example, plasticizer as glycerin, etc. as methylcellulose, the considerable amount of carbon and oxygen remains even after injection molding and the debinder treatment and sintering. It resulted in degradation in magnetic characteristics of a R--Fe--B type permanent magnet, and remains an obstacle to application of a complex shaped part by the injection molding method to a magnetic parts.